The invention relates more particularly to superalloys, in particular those based on nickel and used for making the hot parts of turbomachines, for example the fixed or moving turbine blades of gas turbines which must have excellent resistance to corrosion and oxidization at high temperature, particularly in the presence of molten sodium sulfate from spray and sulfur-containing impurities in the fuel.
One known protection method for this purpose consists in diffusing and/or depositing a layer containing nickel, cobalt, and/or iron aluminide on the surface of the substrate to be protected by using a treatment which includes putting the part to be treated into contact with an additional substance containing aluminium.
One such deposition treatment known as "low activity aluminization" or as "chrome-aluminization" is described in detail in French patent number FR-A-1 490 744, the content of which is incorporated into the present description by reference. In this prior treatment, the additional substance comprises a chromium-based alloy containing 5% to 25% by weight of aluminum and possibly containing 3% to 10% by weight of silicon, with the parts to be treated being put into contact with the additional substance in a finely divided state under a hydrogen-containing atmosphere at a temperature lying between 750.degree. C. and 1200.degree. C.
In order to improve the effectiveness of the aluminization treatment, proposals have been made to precede the aluminization treatment with a predeposition treatment comprising depositing and/or diffusing at least one platinum group metal on the surface of the substrate, with the aluminization treatment then being either low activity aluminization as mentioned above, or else a variant thereof referred to as "high activity aluminization", or else some other type of treatment, e.g. vapor phase aluminization as designated under the reference RT22 by Chromalloy. High activity aluminization differs from low activity aluminization described above in that the metal fraction of the additional substance comprises 55% to 70% aluminum and 45% to 30% chromium, by weight, in that the treatment temperature lies in the range 650.degree. C. to 750.degree. C., and is preferably equal to about 700.degree. C, and in that its duration lies in the range 7 hours to 8 hours and is preferably equal to about 71/2 hours. Such high activity deposition is followed by post diffusion treatment under a non-oxidizing atmosphere (argon or hydrogen) for a period of time and at a temperature which depend on the substrate.
Such a method is described, for example, in the following patents: FR-A-2 071 753, FR-A-2 333 055, GB-A-2 129 017, US-A-3 677 789, US-A-3 819 338, US-A-4 43-9 470, and US-A-3 692 554.
The most commonly used platinum group metal for predeposition treatment is platinum itself which provides a clear improvement in the protection provided by the aluminization treatment. However platinum suffers from the drawback of being very expensive.
Other platinum group metals are mentioned in the above-referenced prior documents, including palladium which is about four times cheaper than platinum. The Applicant has therefore attempted to use palladium instead of platinum, but great difficulties were encountered. It was observed that nickel-based superalloy samples subjected to a predeposition treatment using pure palladium followed by low or high activity aluminization treatment suffered from numerous blisters, leading to poor resistance to corrosion when hot and fragilizing the coating. The Applicant has determined that these blisters are due to a large quantity of occluded hydrogen in the palladium layer which is dissolved in the predeposit while the coating is being made, since hydrogen is very highly soluble in palladium.
The object of the invention is to obtain an aluminide coating modified by a predeposit containing a platinum group metal other than platinum itself, and in particular containing palladium, while avoiding the occlusion of hydrogen or any other gas that could give rise to the phenomenon of blistering.